1. Field of the Invention
The present invention relates to a gallium nitride-based III-V Group compound semiconductor device, and a method of producing the same.
2. Description of the Related Art
In recent years, a great deal of attention has been directed to light-emitting devices utilizing gallium nitride-based III-V Group semiconductors such as GaN, GaAlN, InGaN, and InAlGaN. Such a light-emitting device has a structure in which a layer of an n-type gallium nitride-based III-V Group compound semiconductor, and a layer of a gallium nitride-based III-V Group compound semiconductor doped with a p-type dopant are stacked on a substrate.
In the prior art, the layer of the gallium nitride-based III-V Group compound semiconductor doped with a p-type dopant remains as of high resistivity i-type. Therefore, the prior art device is of a so-called MIS structure. Recently, techniques to convert the high resistivity i-type compound semiconductor layer into a p-type layer has been disclosed in JP-A-2-257579, JP-A-3-218325, and JP-A-5-133189, and thus, it has now become possible to realize p-n junction type gallium nitride-based III-V Group semiconductor light-emitting devices, for example.
As such a p-n junction type gallium nitride-based III-V Group semiconductor devices have been being developed, it has become apparent that an electrode that is formed in direct contact with the p-type semiconductor layer or the n-type semiconductor layer has been encountered with various problems.
Presently, p-n junction type gallium nitride-based III-V Group semiconductor devices have a p-type compound semiconductor layer as the uppermost semiconductor layer due to the restriction imposed on the production thereof. Further, a transparent sapphire substrate is usually used for such a device. Different from a semiconductive substrate used for the other semiconductor light-emitting device, sapphire is electrically insulative. Thus, it is not possible to mount, directly on the substrate, electrodes for supplying a predetermined current to the compound semiconductor layer to cause the device to emit light. A p-electrode and n-electrode must be formed in directly contact with the p-type compound semiconductor layer and the n-type compound semiconductor layer, respectively. The p-electrode is usually formed to cover substantially entire surface of the p-type compound semiconductor layer in order to ensure the uniform application of current to the entire p-type compound semiconductor layer, thereby obtaining uniform light emission from the device. However, since the prior art p-electrode is not light-transmissive, the light emitted from the prior art light-emitting device must be observed on the side of the substrate, which is opposite to the side on which the compound semiconductor layers are formed.
In mounting such a prior art compound semiconductor light-emitting device chip on lead frames, it is therefore necessary to place one such chip on two lead frames, with the p- and n-electrodes facing downward in order to direct upward the opposite side of the substrate on which no semiconductor layers are formed. In short, one device chip must be seated astride the two separate lead frames. In this case, the two lead frames must be set apart from each other at a certain distance in order to avoid electrical short-circuit between the p-type and n-type compound semiconductor layers, which inevitably leads to a large size of a device chip, such as 1 mm square or more. This results in the decrease in the number of chips which can be cut out from one semiconductor wafer. Further, very precise positioning of the two lead frames, and fine etching techniques for the gallium nitride-based compound semiconductors are required.
Next, an n-electrode will be described.
As mentioned above, it is in recent years that p-n junction type gallium nitride-based III-V group compound semiconductor devices can be realized. In the prior art devices of a MIS structure, an electrode utilizes a Schottky barrier with the high-resistivity i-type semiconductor layer, and almost no attention is directed to the n-electrode.
As a material for an n-electrode in the prior art gallium nitride-based III-V Group compound semiconductor light-emitting device, aluminum or an alloy thereof is disclosed in, for example JP-A-55-942. Also, indium is often used. However, it has been found that aluminum and indium can hardly establish an ohmic contract with the n-type gallium nitride-based III-V Group compound semiconductor layer, and tend to degrade by an annealing treatment, losing the electrical conductivity.
In any event, there has been developed, in the prior art, no electrode materials that can establish a sufficiently satisfactory ohmic contact with a gallium nitride-based III-V Group compound semiconductor layers.